It is well known to provide a particulate filter (PF) in the exhaust system of an engine to filter out soot from the exhaust gas flowing from the engine to atmosphere. When the engine is a diesel engine, such a particulate filter is normally referred to as a diesel particulate filter or “DPF”.
A particulate filter (PF) can be damaged during what is known as a “deceleration fuel cut-off” scenario in which during an accelerator pedal tip-out event the fuel is cut-off to the engine of a vehicle to save fuel during deceleration of the vehicle. If the fuel supply to the engine is cut-off when a soot combustion process referred to as a “regeneration event” is active, the percentage of Oxygen in the exhaust gas flowing to the particulate filter will be greatly increased. This increase in Oxygen concentration will often cause runaway combustion to occur in the particulate filter if a large quantity of soot remains in the particulate filter to be combusted during the regeneration process.
When runaway combustion occurs, the temperature within the particulate filter can rise to more than 1000° C. and it is possible to crack the particulate filter, melt the particulate filter substrate or degrade the catalyst washcoat which is present to aid the removal of other regulated emissions (HC, CO or NOx). In an extreme case this overheating condition can result in the particulate filter material combusting which can lead to thermal damage of surrounding components.
A temperature that is likely to result in damage to the particulate filer is an unacceptably high temperature and the particulate filter can be considered to be overheating when subject to such a temperature.